1. Field of the Invention
The present invention relates to a glass-metal joint or connecting device between a glass part and a metal part, to a method of making it, to a solar energy vacuum tube collector and an X-ray tube including the glass-metal joint and to methods of making them.
2. Description of the Related Art
Glass-metal joints or connecting devices are, for example, used in vacuum tube collectors and X-ray tubes. In vacuum tube collectors a vacuum-tight or vacuum-sealed glass-metal bond is required for thermal isolation between the metal absorber tube and the glass tube sleeve.
Working temperatures in collectors produced by concentrated radiation in parabolic channel solar power units reach 400° C. with great local temperature differences. The stresses due to these temperature fluctuations change continuously because of the daily rhythm and temporary cloudy phases.
Currently glass-metal joints or connecting devices, like those described in U.S. Pat. No. 1,294,466, are used in high temperature solar collectors. A very thin metal tongue with a comparatively high thermal expansion coefficient of about 15×10−6 K−1 is fused with a temperature-change-resistant glass with a considerably lower thermal expansion coefficient of about 3×10−6 K−1. The thermal stresses to be expected under thermal loads are then absorbed or picked up by plastic deformation of the metal tongue. The permanently changing stresses unavoidable in a solar collector however frequently lead to mechanical breakdown in these highly loaded regions and to unacceptably high breakage rates of the glass-metal joint of more than 4% per annum. This is a high barrier to widespread introduction of solar technology in the power-engineering field.
These glass-metal joints are so-called unmatched joints, because glass and metal having respectively different thermal expansion coefficients are used to make them.
These techniques are also used for X-ray tubes, as described in U.S. Pat. No. 6,324,870 B1.
So-called matched glass-metal joints or connecting devices are also known from X-ray tube technology. A metal with a comparatively low thermal expansion coefficient is fused or melted together with a glass with an equal or similar thermal expansion coefficient. Currently Schott 8250®, Schoft 8245® and Schott 8447® are used as fusion glass. The fusion glass has the disadvantage that the resistance to water and acid is considerably lower (DIN ISO 719, HGB class 3, pp. 3 to 4) than current borosilicate glass (DURAN®, DIN ISO 719, HGB class 1, p. 1).
Better results have been obtained by fusion of a metal of about 5×10−6 K−1 with a temperature-change-resistant glass of about 3×10−6 K−1 by means of several so-called intermediary or transitional glasses with graded thermal expansion properties. This is very reliable, but it has the disadvantage that the intermediary glass materials are not corrosion resistant, an indispensable prerequisite for a solar collector. In order to make a joint to stable borosilicate glass 3.3 (DURAN®), additional intermediary glass materials are used. A known connection series of this type is: metal (e.g. NiCo2918)-Schoft 8448®-Schott 8449®-Schott 8447®-Schott 8330®-Duran®.
Furthermore these intermediary glasses are not made in tubular form with outer diameters >100 mm for glass technology reasons. The multi-step melting of the intermediary glass as practiced by hand in X-ray tube technology stands in the way of an automated manufacture required for economic reasons in the case of solar collectors. Thus the techniques for connecting glass and metal parts as practiced for X-ray tubes cannot be carried over or used for high temperature solar collectors.
Moreover inclusion of intermediary glasses in the joint increases the engineering work, the disposal rate and processing costs, Furthermore manufacturing methods for glass-metal connecting devices or joints with multiple transitional glasses cannot be automated.